1. Field of the Invention
This invention relates to a lighting circuit for an electric discharge lamp such as a fluorescent lamp, a rare gas electric discharge lamp for near infra-red rays, and other low pressure electric discharge lamps. More particularly, the present invention is concerned with a lighting circuit for such electric discharge lamp which causes it to operate by application of a high frequency voltage having a pause-time period for every half cycle.
2. Description of the Prior Art
First of all, explanations will be given as to the conventional technique for operating the low pressure electric discharge lamp by application of a voltage having a pause-time period.
Japanese unexamined utility model publication No. 4779/1973 discloses a lighting circuit of a construction, in which use is made of a turn-off thyrister provided in parallel with a fluorescent lamp to thereby convert a lamp voltage V.sub.L in a sinusoidal waveform into a plurality of pulse voltage as shown in FIG. 1(a) of the accompanying drawing. This lamp voltage V.sub.L, however, is of commercial frequency, the purpose of which is to reduce a size of a choking coil as a current limiting impedance, hence the prior right invention differs from the present invention.
In the above-described conventional technique, the pulse voltage in the half cycle of the commercial frequency is in one and the same direction. However, Japanese unexamined patent publication No. 36786/1976 discloses an improved technique, wherein the pulse voltage is made in an alternate direction. This prior right invention also aims at size-reduction of the choking coil, the frequency band for use of which is different from that of the present invention, hence no improvement can be attained in the operating efficiency of the low pressure electric discharge tube to be mentioned later.
Further, Japanese unexamined patent publication No. 115078/1976 discloses a lighting circuit for a fluorescent lamp having a capacitive current limiting impedance composed of a serial connection of a capacitor 1 and a choking coil 2 as shown in FIG. 1(b), wherein a switching device 4 is provided in parallel with a lamp 3 to effect short-circuiting of the switching device once every half cycle at the trailing part (those hatch-lined portions in FIG. 1(c)) of the lamp current I.sub.L in a sinusoidal waveform of a commercial frequency. This disclosed art is, however, to facilitate restrike of the electric discharge lamp by increasing a charging voltage V.sub.C of the capacitor 1 at the time of the restriking.
U.S. Pat. No. 3,789,266, which is the closest prior art to the present invention, discloses improvement in efficiency of the abovementioned electric discharge lamp, wherein a high frequency interrupted current having a conduction period T.sub.1 and a breaking period (hereinafter called "pause-time period") T.sub.0, and the current direction of which reverses at every changing of the conduction period and the pause-time period. A device for flowing electric current such as mentioned above is limited to a circuit shown in FIG. 2(b). This circuit is constructed with a bridge circuit, in which transistors 5, 6, 7 and 8 are arranged on four sides and an electric discharge lamp 9 is connected across the diagonal line; another transistor 10 provided in series with the input side of the bridge circuit; and a control device 11 which performs controls of opening and closing of the transistors 5, 6, 7, 8, and 10 to thereby cause electric current to flow as shown in FIG. 2(a).
In such device, however, the voltage to be applied to the transistors 5, 6, 7 and 8 is in a rectangular waveform, at a high voltage portion of which the pause-time period T.sub.0 is formed by the transistor 10 with the consequent disadvantages such that the capacity of the device should essentially be made larger, and that the radio noise is also high. As the results of various studies and experiments made on the operating of the low pressure mercury-vapor electric discharge lamp such as fluorescent lamp with a high frequency voltage having the pause-time period as disclosed in Japanese unexamined patent publication No. 196497/1982 and Japanese patent application No. 110369/1981, the present inventors have discovered that the lamp efficiency can be further improved from the values as shown in the above-discussed U.S. Pat. No. 3,789,266.